How high you are is a measure of fitness. Its like if you have a parcel of land and you specify the latitude and longitude, then there is a certain height of the land for that point. If you specify a particular genome (DNA) for an organism, then there is a certain fitness for that DNA (given a particular environment). The higher the fitness, the more reproductively successful offspring it has.

You can think of its offspring as being at different points on the landscape. Some may be higher (more fit) than the parent, and soon that organism will produce more points, but the lower points not so much. Let's suppose a population which changes its DNA by mutations, and suppose each mutation only gives a small change in fitness. If you look at a whole population of organisms (a whole lot of points on the landscape at various heights) the population will always be pushing upward, as the high points reproduce more and the lower points less. If the population hits a peak, then it cannot push upwards any more. The population will stay around the peak. It won't all be at the peak because it is still mutating, but the points will be gathered around the peak.

Actually there can be mutations which give a large change in fitness too, but these tend to be low on the landscape. Every once in a while one may land near another peak that is even higher than the first peak. Then it forms a population which runs up the new peak and may even out-reproduce and replace those on the first peak.

The fitness landscape is always changing, as the environment changes. A point that is "high" one day may be low a while later. The population is always "searching" for a peak. Organisms that reproduce by mutation have to reproduce quickly in order to keep up. That means they have to be small and simple. If the population sexually reproduces, it can range over a wider landscape, mostly avoiding the low spots, and it doesn't have to reproduce as quickly in order to keep up. That means they can be large and more complicated.

immunogoblin wrote:Or does that just mean that it's easier for the pathogen (in this case the virus) to be transmitted to a new species when the fitness valley is shallow?

Yes. The location of the organisms in a species tend to concentrate at the peak, but mutations and sexual reproduction keep it spread out around the peak, not all at the peak. For a shallow valley, its more likely that this spreading will spread to the upslope of the other peak, and then those organisms will multiply and move up the new peak. For a deep valley, its unlikely that any part of the species that drops that low in fitness will be able to make the jump and make it up the new peak.